The R-T-B based sintered magnet has excellent magnetic properties and thus is used in the voice coil motor (VCM) in a hard-disc drive, various motors such as the motor provided in a hybrid electric vehicle, household electrical appliances or the like. When an R-T-B based sintered magnet is used in a motor or the like, an excellent heat resistance and a high coercivity are required to adjust itself to the working condition at a higher temperature.
As a means to increase the coercivity (HcJ) of the R-T-B based sintered magnet, the heavy rare earth element such as Dy or Tb is conventionally used to substitute part of the rare earth element R mainly employing the light rare earth element such as Nd or Pr so as to improve the magnetocrystalline anisotropy of the R2T14B phase. Up to now, it is hard to manufacture a magnet whose coercivity is good enough for a motor or the like if no heavy rare earth element is used.
However, Dy or Tb has fewer resources compared to Nd or Pr and thus is more expensive. In recent years, the supply of Dy or Tb is rather unstable due to the increasing demand for the R-T-B based sintered magnets with a high coercivity which use these elements in a large quantity. Thus, a coercivity required in an motor application is demanded even if a composition is employed where the use of Dy or Tb is substantially declined.
Usually, the crystal grains in the R-T-B based sintered magnet are micronized so as to increase the coercivity. For example, in the following Patent Document 1, it has shown that the coercivity is improved by making the average grain size of the main phase grains to be 4.5 μm or smaller in the NdFeB based sintered magnet, decreasing the carbon content to 1000 ppm or less in the whole NdFeB based sintered magnet and also making the ratio of the total volume occupied by the carbon-rich phases in the triple junctions to that occupied by the rare earth-rich phases to be 50% or less in the NdFeB based sintered magnet.
In order to micronize the crystal grains in the R-T-B based sintered magnet, the particle size of the finely pulverized particles functioning as the raw material needs to be reduced. However, if the particle size of the finely pulverized particles is reduced, the orientation of the crystals by an applied external magnetic field tends to be more difficult in a pressing step. Thus, some countermeasures may be taken sometimes. For example, the amount of a lubricant to be added in the finely pulverized particles is increased. Alternatively, the finely pulverized particles are mixed with an organic solvent to form a slurry followed by a wet pressing step. However, a problem rises in either case that the content of carbon contained in the R-T-B based sintered magnet obtained after sintering increases and the coercivity decreases. Especially, the smaller the particle size is, the larger the specific surface area is in the pulverized particles. In this way, the carbon content tends to increase so that a technical problem rises that the effect of micronization on the coercivity improvement will not be sufficiently produced.
In Patent Document 1, it has been shown that without the heating for dehydrogenation in the hydrogen storage pulverization, the hydrogen based compound will remain in the particles of alloy powder. In addition, with hydrogen generated therein, the carbon content can be declined in the sintered magnet. However, if a large quantity of hydrogen remains in the particles of alloy powder, a technical problem is there that the volume change with the hydrogen emission during the sintering process becomes larger and cracks are likely to occur in the sintered magnet.
In Patent Document 2, it is described that the coercivity decrease can be inhibited by converting the carbon contained in the R—Fe—B based magnet alloy to carbides with any one or two or more kinds of elements from the group consisting of Cr, Mo, Nb, Ta, Ti, V, W and Zr and then precipitating the carbides in the magnet alloy. However, technical problems are there that the carbides having nothing to do with the magnetic properties need to be precipitated in a large quantity and the residual magnetic flux density decreases in the obtained magnet.